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Impairment Charges and Mine Closure Costs (Details) - Predecessor - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2016
Mar. 31, 2016
Jun. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items]      
Other-than-temporary impairment loss on available-for-sale securities   $ 2.9  
Severance costs     $ 7.2
Curtailments     $ 3.6
Royalty Agreements      
Impaired Long-Lived Assets Held and Used [Line Items]      
Prepaid royalties   3.4  
Kentucky      
Impaired Long-Lived Assets Held and Used [Line Items]      
Net book value of idled facility   $ 74.1  
Longview, Washington      
Impaired Long-Lived Assets Held and Used [Line Items]      
Other-than-temporary impairment loss on available-for-sale securities $ 38.0